I follow a whole range of people, including some TV people and some researchers, as well as organisations.
Here are some examples (some of them are more prolific tweeters than others), many of them not directly related to birding, but do have some link to natural history:
@UKNHB - Nature Bloggers
@Birdingblogs - Birding Blogs
@ARKive - Arkive
@LinneanSociety
@WoodlandTrust
@Buzz_dont_tweet - Buglife
@savebutterflies - Butterfly Conservation
@CornellBirds - Cornell Lab of Ornithology
@_BTO
@BTO_Scotland
@GrahamBTO - Graham Appleton of the BTO
@miketoms - Mike Toms BTO bod
@BirdTrack
@wildliferanger1 - Adam McClure - BTO regional rep and red kite officer
@FI_Obs - Fair Isle Bird Obs
@CopelandBirdObs - Copeland Bird Obs.
@IofMayBirdObs - Isle of May Bird Obs.
@FaunaFloraInt - Fauna and Flora International
@IUCN
@AmazingSpecies - IUCN Red List
@EDGEofExistence - highlights species on the edge of extinction
@graemelyons - naturalist and ecologist
@royhdennis
@TonyJuniper
@WildlifeG - Gillian Dinsmore, ecology student at Stirling Uni
@GrrlScientist - prolific tweeter, including to regular column on the Guardian about nature books
@Laelaps - Brian Switek, science writer (mainly on dinosaurs)
@urbanbirder
@ScottishBirding - SOC
@JohnMuirTrust
@NaturalScotland
@ScotWildlife - SWT
@TheFallsOfClyde - The Falls of Clyde SWT team
@ScottishBeavers - Scottish Beaver Trail Knapdale
@RSPBScotland
@RSPBGlasgow
@RSPBLochwinnoch
@RSPB_NEScot
@StuartHousden - RSPB Scotland director
@MarkAvery - former RSPB director, now author and blogger
@BirdLife_News - BirdLife International
@BBC_Springwatch
@michaelastracha
@ChrisGPackham
@BillOddie
@TVsSimonKing
@gordonjbuchanan
@MayaPlass
@rtaylorjones - Richard Taylor Jones, filmmaker featured regularly on Springwatch
@bugboybaker - Nick Baker
@BBCNature
@StephenMoss_TV - author and producer
@iron_ammonite - Paul Williams, filmmaker
@DougAllanCamera - Frozen Planet cameraman, one among many series
@GavinThurston - cameraman on many of Attenborough's series
@adamscott5 - producer of many of Attenborough's recent series
@DilgerTV
@JohnAitchison1
@nigpope - Nigel Pope, producer of the recent Hebrides series
@Elizabeth0White - filmmaker with BBC Natural History Unit
@SeabirdCentre - Scottish Seabird Centre North Berwick
@20_20_VISION - check out the photos on their site
@NatureInShet - Nature in Shetland, along with below, great for wildlife news from Shetland
@ShetlandWild
@HughHarrop - owns Shetland Wildlife
@SNH_Tweets - Scottish Natural Heritage
@WHALES_org - WDC
@sealifesurveys - Whalewatch Scotland
@HWDT_org - Hebridean Whale and Dolphin Trust
@mcsuk - Marine Conservation Society
@saveourseas - Save Our Seas Foundation
@SaveScotsSeas - Save Scottish Seas
@iSpot_uk - Open University citizen science project
@Earth_News
@biofreshproject - BioFresh promoting science and conservation of freshwater environments
Some people on BirdForum (although I don't think any follow me!)
@FatPaulScholes1
@tom_mckinney
@ebirder
@birdforum
You can also follow me on @JTweedie